There is no definitive way to know who your contacts are talking to. In fact, it is normal that from time to time a "100% reliable" method of discovering who our WhatsApp contacts are communicating with pops up on the Internet, but they really are a scam.
Some methods take advantage of small vulnerabilities in the app (which are quickly corrected) or use application-like mods that supposedly monitor our contacts. Methods that, by the way, are highly questionable. These applications cannot know who our contacts are talking to.
Some other apps can track your contacts' last connection times (which means you will need to have your last connection time enabled, otherwise it will not work) In fact, these applications usually have a short life because they violate countless terms and conditions. This kind of apps works in a very simple way, after entering the contacts we want to "watch", the application checks the time when several of these contacts were online and cross-checks the data. When the last connection time of two contacts coincides, or they are very close in time, the application assumes that it is possible that they were talking to each other. The application does not give any kind of guarantee regarding if two contacts are talking to each other since it does not read or capture messages.
If at any time you see any application that promises to get you this information, be skeptical. Most of them are paid to use and do not work or are loaded with advertising and we do not know who we are giving our phone number and our contact list to, or for what purpose. So, at this point, we recommend you not to use any of them.